Aims: This study aimed to evaluate the expression levels of virulence gene regulators (luxR and toxR) and virulence factors (serine protease, metalloprotease and haemolysin) in luminescent and nonluminescent isogenic Vibrio harveyi and Vibrio campbellii. Method and Results: Nonluminescent variants have been reported before to become dominant in cultures of luminescent vibrios when grown under static conditions in the dark. Wild-type V. harveyi BB120, V. campbellii LMG 21363, quorum sensing mutants of V. harveyi BB120 and their previously reported nonluminescent isogenic counterparts were used in this study. The expression level of the virulence genes srp serine protease, vhp metalloprotease and vhh haemolysin, the quorum sensing master regulator gene luxR and the virulence regulator gene toxR in isogenic luminescent and nonluminescent strains were quantified using reverse transcriptase real-time PCR. These experiments revealed that the nonluminescent strains produced lower levels of the quorum sensing master regulator gene luxR and the vhp metalloprotease gene (which is known to be regulated by quorum sensing). Finally, challenge tests with gnotobiotic brine shrimp (Artemia franciscana) larvae revealed that the nonluminescent strains are less virulent than their luminescent isogenic counterparts. Conclusion: Nonluminescent variants of V. harveyi and V. campbellii strains produce lower levels of the quorum sensing master regulator gene luxR and the vhp metalloprotease gene and are less virulent to brine shrimp than their isogenic luminescent counterparts. Significance and Impact of the study: These results indicate that adaptation of luminescent vibrios to specific growth conditions that result in a dominant nonluminescent phenotype is accompanied by a decreased adaptation to a host environment because of altered virulence gene regulation.